1. Field of the Invention
The present invention relates to a transmission bandwidth sharing system, e.g., CBR communication system, etc., and more particularly to a transmission bandwidth sharing apparatus used in an ATM exchange for sharing one CBR (Constant Bit Rate) bandwidth on an ATM (Asynchronous Transfer Mode) line among a plurality of ATM terminals in a multimedia network or the like, and a method for the same.
2. Description of the Related Art
FIG. 1 is a diagram for explaining an ATM cell transmission in CBR communication.
When a CBR bandwidth is set up in an ATM exchange, valid cells (shaded cells in FIG. 1) are sent out from the ATM exchange onto an ATM line at a constant interval a, as shown in FIG. 1. The required CBR transmission bandwidth (CBR setup bandwidth=n Mbps) is determined based on the cell interval a in the peak traffic that uses the bandwidth.
In recent multimedia communication systems, compression techniques are often used to increase the transmission efficiency of voice, video, and other information. In the case of an ATM terminal using a compression technique, input data is first compressed and then assembled into cells for transmission over an ATM network.
FIG. 2 shows a schematic representation of a process in which video information is compressed and assembled into ATM cells by an ATM terminal for transmission over an ATM network.
In FIG. 2, a video signal 2, for example, a 6-MHz analog television signal, from a video terminal 1 is input to an ATM terminal 3. A compression mechanism 4 in the ATM terminal 3 converts the input signal into a compressed signal 5, such as an MPEG-2 compressed signal, and an ATM conversion mechanism 6 converts the compressed signal 5 into ATM cells 8 for transmission onto an ATM line 7.
Since the compression mechanism 4 compresses the video signal 2 utilizing the regularity of the video data, it requires buffering the data for a certain period of time. The compressed data is then assembled into a frame of fixed length and transferred to the ATM conversion mechanism 6. Accordingly, the compressed signal 5 is output in a bursty manner at fixed intervals of time, as shown in FIG. 2. Further, since the ATM conversion mechanism 6 converts the thus received frame signal (compressed signal 5) into cells, the interval at which the ATM cells 8 are sent out on the ATM line 7 is not constant, though it has certain regularity.
As previously stated, when a CBR bandwidth is set up in an ATM exchange, valid cells are output from the ATM exchange onto the ATM line at constant intervals of time. On the other hand, when information compression is performed at the ATM terminal, since data is buffered for a certain period of time before applying the compression, the interval between the valid cells sent out on an ATM network loses regularity in terms of the interval between adjacent cells, though there is a periodicity defined by the buffering time.
Generally, for transmission of video information, it is desirable to use CBR as a service category in view of its real time requirement, cell loss prevention, etc. When an ATM terminal using a compression technique is accommodated in an ATM exchange providing CBR transmission between stations, the bandwidth to be assigned to the CBR transmission is determined based on the peak cell rate (the cell rate in a time segment where the cell interval is short) of non-constant signals from the ATM terminal.
FIG. 3 shows the compression process of FIG. 2 in comparison with the CBR line of FIG. 1.
In FIG. 3, the valid cells (shaded cells in FIG. 3) output from the ATM terminal 3 are subjected to compression at the ATM terminal 3 at a cycle d. In the illustrated example, the compression cycle d is twice the valid-cell cycle a in FIG. 1 (d=axc3x972). On the other hand, the shorter valid-cell cycle b is ⅓ of the valid-cell cycle a in FIG. 1 (b=⅓a).
In this case, therefore, the required CBR setup bandwidth is three times the bandwidth in FIG. 3 (CBR setup bandwidth=3n Mbps), but the number of valid cells actually transmitted within one compression cycle in FIGS. 3 is 2, which is equal to the number of valid cells transmitted within the same length of time in FIG. 1, i.e., within two valid-cell cycles (axc3x972). As a result, in the illustrated example, two thirds of the CBR setup bandwidth are occupied by invalid cells (blank cells in FIG. 3), as shown in the right-hand side of FIG. 3.
In this way, the prior art has had various problems affecting transmission efficiency of CBR; for example, 1) invalid cells are sent out on the CBR channel between ATM exchanges during the period that compression is performed at the ATM terminal side, and 2) as a result, a CBR bandwidth greater than the necessary throughput has to be reserved, which limits other traffic on the network.
In view of the above problems, it is an object of the present invention to achieve efficient utilization of a CBR bandwidth by preregistering valid-cell transmission timings and invalid-cell transmission timings when setting up the CBR bandwidth at an ATM exchange, and by allowing the same CBR bandwidth to be shared by a plurality of terminals based on combinations of the timings so that valid cells can be transmitted at timings where invalid cells would otherwise be transmitted.
It is also an object of the present invention to achieve efficient utilization of the CBR bandwidth when an additional terminal is connected to the ATM exchange, by additionally setting in the existing CBR bandwidth the valid-cell transmission timings and invalid-cell transmission timings for the additional terminal.
According to the present invention, there is provided a transmission bandwidth sharing apparatus accommodating a plurality of terminals and having means for allocating, based on bandwidth information of one transmission channel bandwidth being constant and terminal information relating to each of the plurality of terminals, a timing in an invalid signal transmission period of one of the terminals for use as a transmission timing of a valid signal from another one of the terminals, thereby allowing the one constant bandwidth of a transmission channel to be shared among the plurality of terminals.
The transmission bandwidth sharing apparatus comprises: a bandwidth management table for registering a bandwidth and information relating to the bandwidth for each identified transmission channel bandwidth; a bandwidth sharing management table for registering, for each identified bandwidth, one or more terminals that use the bandwidth and information relating to the one or more terminals; a signal transmission timing management table for registering valid signal transmission timings of the registered terminals for each identified bandwidth; and transmission sharing means for allocating, based on the bandwidth information and terminal information, a timing in an invalid signal transmission period of one of the registered terminals for use as a transmission timing of a valid signal from another one of the registered terminals, thereby enabling a plurality of communication connections to be established by sharing the identified one transmission channel bandwidth.
The transmission bandwidth sharing apparatus further comprises transmission means for transmitting all or part of the management tables to another transmission bandwidth sharing apparatus at a remote end of the communication, and the transmission sharing means sets up only one communication connection within one transmission channel bandwidth when the sharing of the transmission channel bandwidth is not specified.
The transmission bandwidth sharing apparatus and the terminals are ATM devices, and the bandwidth information includes bandwidth identifying information, a peak cell rate to be shared by the terminals, and the total number of valid and invalid cells contained therein, while the terminal information includes the peak cell rate of each of the terminals and the number of valid cells contained therein.
The transmission sharing means allocates timings in invalid cell signal transmission periods to terminals when the sum of the peak cell rates of the terminals is within the peak cell rate to be shared by the terminals and when the sum of the valid cells of the terminals is within the total number of cells contained in the peak cell rate to be shared by the terminals.
According to the present invention, there is also provided a transmission bandwidth sharing method, comprising: registering a bandwidth for each identified transmission channel bandwidth being constant; registering, for each identified bandwidth, one or more terminals that use the bandwidth; allocating, for each identified bandwidth, a timing in an invalid signal transmission period of one of the registered terminals for use as a transmission timing of a valid signal from another one of the registered terminals; and allowing said other one of the registered terminals to transmit a valid signal at the valid signal transmission timing allocated thereto, whereby a plurality of communication connections are established within the identified one transmission channel bandwidth.
The method further includes transmitting all or part of registration information obtained by the registration to another apparatus at a remote end of the communication in order to share transmission channel information, and making said other apparatus change the transmission channel information upon receipt to information that matches said other apparatus. The method also includes setting up only one communication connection within one transmission channel bandwidth when the sharing of the transmission channel bandwidth is not specified.